Britain's top brass are very unhappy. And so they should be. They have already accepted cuts in the number of navy ships, army tanks, and RAF planes. Now deep cuts in the number of sailors, soldiers and air force personnel are being forced upon them.

Although their predicament has not been helped by pusillanimous ministers, they have not covered themselves in glory. Service chiefs resorted to squabbling among themselves as it became increasingly clear that their spending on weapons systems Britain could not afford, and in some cases did not need, would end in tears.

Libya has given the RAF a temporary lifeline, although it is hard to see where it would get such an easy ride again. The navy fired the odd Tomahawk cruise missile from its submarines at the north African state and seized a rare opportunity to fire the guns of its destroyers, but the army scarcely got a look in.

And there's the rub. What will the regular army do when its combat role in Afghanistan comes to an end in 2014? The lack of a convincing answer has left the generals vulnerable to even deeper cuts than those due to be officially announced on Thursday. Will hundreds, let alone thousands, of British troops be deployed ever again to foreign fields? Humanitarian missions abroad, natural disasters at home may be the future.

General Sir Peter Wall, head of the army, observed recently: "Wars pick us, we don't pick them." That has been a doubtful assertion ever since the cold war – even more so after Libya. And as General Sir David Richards, chief of the defence staff, has himself suggested, future warfare will in any event be very different. States may use guerrilla or terrorist groups as proxies, as Iran and Syria have used Hamas and Hezbollah.

The idea of a British army ever again confronting a national army of a hostile state, or groups of insurgents, in the open is anachronistic. Electronic warfare, cyber attacks, small groups of commandos and special forces will take the place of traditional army units, just as pilotless drones will take the place of conventional aircraft.

Few serious commentators, outside or inside the Ministry of Defence, now deny that last year's security and defence review was eccentric and bizarre, leaving the navy with two costly aircraft carriers, neither of which will carry aircraft for a decade, one due to be mothballed as soon as it is completed. The government excluded from its review plans to replace the Trident nuclear missile system, described even by Tony Blair in his autobiography, A Journey, as "non-existent in terms of military use".

Ministers do not seem to mind cuts in the number of armed forces personnel (or civilian MoD employees) while apparently insisting that Britain's forces must be prepared to fight in any possible future conflict. David Cameron insisted, when questioned this summer by senior backbench MPs, that Britain had "a full-spectrum defence capability" – a claim denied by all three heads of the armed forces.

"I tell you what, you do the fighting and I'll do the talking," the prime minister told the service chiefs this summer after what he regarded as another bout of whingeing about the pressures they faced. Whatever bravery their troops have shown in Afghanistan, the generals have not fought their internal battles at all well. More cuts will be on their way as the army faces an unprecedented struggle for its existence as a traditional fighting force.